A number of respiratory protective devices are fitted with filter box assemblies and/or filter units. The filter box assemblies and/or filter units are ordinarily connected to a facepiece, such as a half-mask or a mouthpiece, which provides an effectively airtight connection between the breathing zone or mouth of the wearer of the device and the filter box assembly/filter unit. This ensures that the user can only breathe air which has been filtered to remove contaminants including, for example, toxic gases and particulates.
As noted above, respiratory protective devices often need to be used when there is the presence of airborne hazardous substances. However, in such situations many users find that the breathing resistance of the devices makes them extremely uncomfortable to use, to the extent that some would-be users refuse to wear such equipment. Such situations most typically occur where the users are required to carry out high energy work; where the users are unfamiliar with wearing respirators or protective devices; or where the users are scared or anxious (for example, when using the device for self-rescue).
If users consider the devices to be too uncomfortable, they may refuse to wear them or they may wear the devices incorrectly in an attempt to minimise discomfort. In such cases the wearers may be provided with no, or substantially reduced, respiratory protection against the airborne hazardous substances of concern.
Attempts have been made to provide respiratory protective devices with breathing resistances that are acceptable to the majority of wearers in the majority of situations. The breathing resistance in such devices is based on national and international standards, which specify upper limits for breathing resistance. However, these standards were drafted and set several decades ago. Thus, current such limits are based on what could have been achieved using historical filter technologies, albeit they were current when the national and international standards were drafted. It would, of course, be of more use to design modern filters based on physiological data as to the subjective acceptability of the imposed breathing resistances of respiratory protective devices.
In order to minimise breathing resistance, the size of filters used in respiratory protective devices can be increased and/or multiple filters may be used. Although devices with large or multiple filters will have a lower breathing resistance than devices fitted with single filters of normal size, the use of larger or multiple filters increases the bulk and weight of the devices. Such increased bulk and weight is disadvantageous, particularly for devices which must be carried on the body, or stored in large numbers close to sites of potential use.
Furthermore, there is an express need in some instances for devices to be even more compact than existing devices. For example, there are instances where respiratory protective devices are required to be compact in order to be inconspicuous or easily carried about the person for self-rescue.
Many respiratory protective devices contain both particulate filters and gas (or vapour) filters. Both the particulate and gas filters used have to meet technical requirements as to their protective performance and/or capacity against the agents of concern. Therefore, such devices have a set minimum volume of gas and vapour absorbents/adsorbents, and a set minimum area of particulate filter media that is required such that they comply with the relevant national or international standards.
In an attempt to address some of the problems highlighted above, a number of devices have used “double-sided filters”, i.e. filter assemblies where two or more filter sub-units feed into a single clean air channel, as a means of minimising breathing resistance while enhancing the protective performance of the particulate filters. However, although technically effective, such double-sided filters can be awkward to manufacture to the required standards of quality; particularly if the gas and vapour filter beds are of shallow depth, e.g. of less than about 10 mm depth.
Conventional gas and vapour filters incorporate loose granules of adsorbents/absorbents. When using such filters, it is necessary to apply a force to the granule bed to lock the granules in position so that the granules do not rub against each other, abrade and/or slump, so forming voids through which gases or vapours can pass without being retained by the adsorbents/absorbents. Such force is usually generated by incorporating some form of spring mechanism. This makes such filters difficult to manufacture and, inevitably, adds to the overall bulk. This is particularly the case when building a filter assembly with more than one filter unit.
There is therefore a need for a simple and reliable means of manufacturing compact double-sided, or other multi-sided, filter box assemblies that contains gas (vapour) and particulate filters.
There is also a need for a compact filter unit having a gas and a particulate filter layer at least, and a simple and reliable means of manufacturing such a compact filter unit and an associated filter box assembly.
Therefore, it is an object of the present invention to obviate, or at least mitigate, at least some of the drawbacks associated with the prior art.
Further aims and objects of the invention will become apparent from a reading of the following description.